Modelling Groundwater Flow in Hard Rock Environments Alexander Renz, DHI, Munich, Germany

Part 1

Representing Complex Geology

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Complex

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3D Groundwater Model

3D Groundwater Model

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Multi-Faulted Regional Groundwater Model, Switzerland

3D Groundwater Model

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• Continuous Layers

• Vertical Walls

Unstructured Meshes

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Example of teaserFollow the guide from the Branding SharePoint page to learn how to change the pictures

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All Problems solved?

Traditional Layered Meshes

Creating Computation Meshes

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Mathematical Approach

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• Finite Differences Method • Finite Element Method

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2.5 D Mesh (Layered Mesh)

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2 Triangles →

1 Quadrangle

Element Deactivation

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• Create Caveats in Mesh, e.g.

for pit mines or tunnels

• Permanent or temporary

Element Deactivation

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• Remove unwanted layers

(Top / bottom only)

Steep Interfaces

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• moderate inclination:

Layer Elevation

• steep inclination:

step layout

Layered Meshes with local remeshing

Creating Computation Meshes

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Pinching

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Topography

Pinching

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Local Re-Meshing

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Non-Layered (Fully Unstructured) Meshes

Creating Computation Meshes

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Non-Layered Meshed

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• Geological contacts (volumes)

• Insets such as well locations

Non-Layered Meshed

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Legacy Approach

Structural Geologic Model

Computation mesh for numerical

model

Interfaces of geologic units

Non-Layered Mesh

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Interfaces and geologic units

Non-Layered Mesh

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Tetrahedral-meshed geologic units

Non-Layered Mesh

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Clipped tetrahedrons

Non-Layered Mesh

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Non-Layered Mesh

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Full tetrahedral mesh

Challenges

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Piecewise Linear Complexes (PLC)

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http://wias-berlin.de/software/tetgen/plc.html

The Problem of Self-intersecting Surfaces

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Closed and Consistent Volumes

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Piecewise Linear Complexes (PLCs)

DHI In-House Solution with TetGen

Example Case

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Consideration of Fault Offsets

Consideration of Fault Offsets

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Consideration of Fault Offsets

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• Case 1: Offset at isolated fault

• Solution: Record and sort all elevation data at faults

+ applicable for isolated faults

- Not applicable for isolated faults

Consideration of Fault Offsets

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• Case 2: Intersecting Faults

Grouping of elements to sectors

+ can represent more generell complex settings

Consideration of Pinchouts

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• Pinchouts and outer model hull

Closing of Remaining Gaps

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• Ensure closed volumes

Regional Markers

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• Bookkeeping for later numerical model

Entwicklung der Methodik

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• Result: Set of 3D face objects consistently enclosing all separable

geologic volumes

• ~ 700.000 points

• ~ 500.000 faces

Geologic Units

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VIDEO

Geologic Units

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VIDEO

Face Objects

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• red: Outer Border

• Light green: Faults

• blue: pinch-outs

• Other colors: geologic contacts

Resulting Mesh

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Carbonate Layer at Depth

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CGAL Meshing GeneratorComputational Geometry Algorithms Library CGAL

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CGAL Library

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• Computational Geometry Algorithms Library CGAL

• Open Source (www.cgal.org)

• Integrated in Geomodeller (Intrepid Geophysics)

Pilot Model of Lusitian Over-Thrust

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• Geological model created in

3D Geomodeller

• Incl. Major geologic units

• Incl. vertical faults and

Lusatian overthrust

Unstructured mesh generated

Tetrahedron mesh created using CGAL

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Faults require a finite thickness

• Conduit

• Barrier

Handling of Faults

Local refinement

Handling of Faults

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Nodal connections

Test24.fem

Mesh Refinement

Accuracy vs. Computational Speed

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more refinement more refinement

Elements: 669.910

Nodes: 114.274Elements: 723.628

Nodes: 124.077

Elements: 1’201.954

Nodes: 201.930

Optimum for Geometric Representation

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Optimum for Computation

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Finally: What are the Benefits?

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Flexibility

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Thank you!are@dhigroup.com

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